1. Field of the Invention
The invention relates to mixing and digital conversion of several analog signals, in particular in so-called `multimedia` computer systems, where input analog signals, for example audio signals, arrive from a variety of sources and are later exploited digitally.
2. Discussion of the Related Art
FIG. 1 shows a conventional signal processing system suitable for use in multimedia applications. The circuit is arranged as an input path 2, an output path 4 and a feedback path 6. A number of analog input signals I1, I2, I3, I4 are each connected to a corresponding variable gain analog amplifier G1, G2, G3, G4. Each of these amplifiers G1-G4 may provide a level of amplification or attenuation, according to a control signal which may be supplied by a host microprocessor 10, using a data or command bus 20. The amplified or attenuated signals I1', I2', I3', I4' are each supplied by the respective amplifier to a respective mute circuit M1, M2, M3, M4. The mute circuits M1-M4 are also controlled by the data or command bus 20. The outputs of the mute circuits M1-M4 are connected as inputs to a first analog mixer 22. The output of the analog mixer 22 is connected to an analog-to-digital converter 24, whose output is connected to a data bus 26.
The digital signals from analog-to-digital converter 24 are the mixed and digitally converted representation of all the input signals I1, I2, I3, I4, according to ratios set by the amplification or attenuation of amplifiers G, and according to the passing or blocked state of mute circuits M1-M4.
The data bus 26 is also connected to the input terminals of a digital-to-analog converter 28. The output of the digital-to-analog converter 28 is connected to an input of a mute circuit M5. The output of this mute circuit M5 is connected to a second analog mixer 32. This second mixer also receives signals I1', I2', I3', I4' via mute circuits M1', M2', M3', M4', which are controlled by data or command bus 20. The output of mixer 32 is supplied to a variable attenuator A, which attenuates the signal from the mixer 32 to a level suitable for provision as an output signal O. Variable attenuator A is controlled by data or command bus 20. Output signal O is provided to a mute circuit M6, whose output is connected as an input to first mixer 22.
Each mute circuit is operable to either pass the signal present at its input, or to block this signal and provide no analog signal as an output. The analog mixer circuits 22, 32 act as adders, and add together the connected input signals. This is done in the analog domain, and the resulting mixed signal is converted to a digital representation later.
The digital representation of the output of the analog mixer 22 may then be subjected to any required signal processing operations by the microprocessor 10.
Before being supplied as an output signal O. the digital representation is converted back to an analog form by digital-to-analog converter 28, and may be mixed with a selection of the input signals, chosen by selecting the states of mute circuits M1', M2', M3', M4'. By placing mute circuit M5 in its blocking state, an output signal O comprising a mixed version of one or more input signals I1, I2, I3, I4 may be supplied, without the use of the digitally represented signal.
By placing mute circuit M6 in its passing state, the output signal O may be fed back into mixer 22 for further processing.
As the input signals I1 to I4 may be provided by different sources, the gain or attenuation of each amplifier G1 to G4 must be individually adjusted, to ensure that each signal I1'-I4' is at an adapted level for the mixer 22 and the analog-to-digital converter 24, to avoid exceeding the maximum input of the converter 24.
The variable attenuator A is required to ensure that the output signal O is at a suitable level for the circuitry which receives it. It also allows the output signal O to be fed back into the first mixer 22 without drowning out the other signals I1' to I4'.
Gain control and mixing of the signals is done in the analog domain. The dynamic range of this circuit is limited, both by the supply voltage to the mixer, and the full scale range of the analog-to-digital converter 24. This is a problem when several signals are summed together, hence the need for the variable gain amplifiers G1-G4. Also, these variable gain amplifiers G1-G4 ensure that a strong signal (e.g. an electronic keyboard output) does not drown out a weaker signal (such as a signal from a microphone). Noise is generated by each circuit block, and is added by the mixer 22, so that the total noise content of the signal produced by the mixer 22 may be very high. This noise cannot be filtered out, and can cause errors greater than the quantization level of the digital conversion. Zero crossing detection of signals is desirable for performing gain control, but is difficult to perform in the analog domain. Crosstalk between several analog signals all being treated on a same integrated circuit is often also a problem. This may be directly between signals, or via supply lines.
Furthermore, it may be desirable to cancel the DC offset of each signal before performing gain control. This also is difficult to perform in the analog domain.
An object of the invention is to provide a processing and mixing circuit for a number of analog signals, which occupies a particularly small semiconductor surface area.
Another object of the invention is to provide such a circuit which may avoid or reduce the problems of crosstalk, zero crossing detection, DC offset cancellation, gain control and dynamic range limitation.